


A Long Time Coming (W.D. Gaster/Sans)

by EsperAqua



Category: Undertale (Video Game)
Genre: Blow Jobs, Ecto-Penis (Undertale), Ecto-Tongue (Undertale), Eventual Smut, Fellatio, First Time Blow Jobs, Fluff, Fluff and Smut, Gaster / Sans, Good W. D. Gaster, Happy Sans, M/M, Masturbation, Pain, Pre-Accident W. D. Gaster, Sans / Gaster, Sans / W.D. Gaster, Sans x Gaster, Sans x W.D. Gaster, Scientist Sans, Scientist W. D. Gaster, Shy Sans, Soul Sex, Suffering, Suffering Sans, The Core (Undertale), True Lab (Undertale), W. D. Gaster is not related to Skelebros, soul play, w.d gaster / sans
Language: English
Status: Completed
Published: 2016-07-23
Updated: 2016-12-11
Packaged: 2018-07-26 03:54:04
Rating: Mature
Warnings: No Archive Warnings Apply
Chapters: 8
Words: 14,390
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/7559107
Author URL: https://archiveofourown.org/users/EsperAqua/pseuds/EsperAqua
Summary: <blockquote class="userstuff">
              <p>Sans becomes Doctor W.D. Gaster's lab apprentice, his ultimate dream job, but despite everything things are not always what they seem, and eventually it becomes apparent that Doctor Gaster has other plans in store for him.</p>
            </blockquote>





	1. A New Job

**Author's Note:**

> If you like this and want to stay up to date on new chapters, be sure to bookmark or follow me at my Tumblr at http://esperaqua.tumblr.com/ (occasionally NSFW)

The day was finally here, the day Sans would start his new job as the Royal Scientist's new apprentice. For years he had been scouring the dump of the Underground, searching for all the scientific books and papers he could find, resulting in many late nights of "boning" up on all he could regarding anything related to science matters. He had a surprising knack for it, everything just seemed to click into place, with mathematics and physics being two of the subjects he truly excelled on.

When he was called in for an interview, he couldn't have been any more nervous. He recalled sitting in the chair opposite his interviewer's desk, practically sweating bullets as he waited on his test scores to be revealed. His interviewer, Doctor W.D. Gaster, head of the infamous Core Project and the Royal Family's top scientific mastermind, sat in silence as his eyes scanned over the papers he held in his oddly punched hands. Occasionally he would break away from them to sip at his coffee mug and look over Sans questioningly through oval shaped glasses taped to the sides of his skull, but he would eventually go back to reading the papers with a deep hum or curt laugh.

After what seemed like an eternity, Dr. Gaster placed the papers on his desk, steepled his long bony fingers together and leaned forward to look down at the smaller sweating skeleton sitting before him. Sans gulped and wrung his hands together, looking up at the doctor with fear in his eyes.

"Sans, your test scores are... outstanding." Gaster said softly with a bit of surprise in his voice. Sans audibly sighed out, nearly collapsing back in the chair, but quickly moved to compose himself in front of the doctor. "You actually have the highest test scores of anyone that has ever interviewed with me. How though? How can you, a skeleton from a small town in the middle of nowhere, know so much about all these subjects you were tested on?"

Sans fidgeted in his chair and laughed nervously. "I guess I'm not as thick skulled as I seem." Knocking on the side of his skull, he gave the doctor a weak smile. " Actually, I've been studying for years to get here. It has always been a dream of mine to work at the Lab, no bones about it. Heh."

Doctor Gaster visibly winced at Sans' puns but held back a smile he could feel was just underneath the surface. He was not usually one to show much emotion in front of potential employees, but this one had prospect and seemed to be quite eager to please. "I see. Well then, based on your test scores and your, um, enthusiasm, I'd like to offer you the position of my apprentice. You will assist me with my daily projects, as well as work with me on improving the stability of the Core. Do you accept my offer?"

Sans gripped onto the arm rests of his chair and shook with excitement, his bones rattling audibly. "Y-yes, of course! It would be an honor, it's all I've ever wanted!"

"Good. I will send someone in for you to complete your paperwork. You can start next week." Doctor Gaster got up from his seat and walked to his door, leaving Sans wide eyed and still gripping at the arm rests on his seat with excitement. He opened the door, paused, and turned back to Sans looking at him with intensity in his glowing white eyes. "And Sans... Wear purple on your first day. It's my favorite color."

Sans blinked as he watched the doctor then leave the room, his white lab coat trailing behind him. Sighing out in relief, he slumped back into the chair, wiping away the sweat that had accumulated on his brow. 'Shit that was intense', he thought as he allowed his body to relax, 'I can't believe I did it, I really did it! I can't wait to tell Paps! ...Wear purple, huh? Odd request, but I'll do anything the doctor wants!'

 

Anything the doctor wants. That was Sans' manta as he got ready for work, putting on the very best black dress slacks and black button down shirt that Papyrus had washed and ironed for him the night before. He slid a silk purple tie around his vertebrae and began to fumble around with the garment, unable to tie it up properly. He growled and temporarily gave up on it, walking out of his room to Papyrus' room next door, knocking on the door to see if he could help.

"Hey Paps, you there? I, uh, need a little help."

The sound of typing could be heard from Papyrus' room, but it quickly stopped shortly after Sans knocked on the door.

"Absolutely! Come on in!" Papyrus yelled, thinking his brother couldn't possibly hear him normally with the door closed.

Sans chuckled at his brother's volume and opened the door. "Hey bro, I can't get this thing to work. It's KNOT really working out for me..."

Papyrus rolled his eyes and got up from his computer, walking over to Sans. "Really, Sans? 'Knot' working out? I can take all your horrible puns, but I certainly hope you'll cool it when you are working with the Royal Scientist, as he probably won't be as understanding of you as I am."

"Heh, yeah, well t- " Sans stopped mid-sentence as Papyrus gave him the stink eye while assisting him with his tie. "S-sorry, bro, I'm just nervous. Big day and all."

Papyrus finished the correct knot, a Double Windsor, and held Sans by the shoulders, his sour face turning into a smile as he looked his brother over. "Oh Sans, I know you will be fine. You've prepared so much for this, you've got this! I believe in you!"

Sans grunted as Papyrus suddenly pulled him in for one of his patented "skele-squeezes", and closed his eyes as he felt a bit of the edge wash away. After a few moments Sans patted Papyrus on the back, pulling away reluctantly. "Come on Paps, you are going to wrinkle the fabric."

"Ah! I am! I'm sorry!" Papyrus released his brother and backed off with his arms in the air with a smile. "You had better get going soon though, you wouldn't want to be late on your first day."

"Nope, I sure would _knot_..." Sans smirked smoothed out his shirt and smiled back at Papyrus, who was gritting his teeth together at his brother's re-used pun. "I'll see you later, don't wait up for me if it gets too late, we can talk about everything in the morning."

"I was going to make pancakes, but I think after that one, you are on your own..."

Chuckling, Sans winked at his brother. "You know you like my puns..."

"No, I HATE them!"

"Heh, sure bro, _tie_ know you do. Welp, I'll see you later. Wish me luck." And with that Sans teleported out of the room with a "pop", leaving Papyrus to once again roll his eyes at his brother's ridiculousness.

  
Sans arrived at the front door of the Lab and instantly began sweating at the incredible heat of the Hotlands. He quickly rang the buzzer and announced his arrival, eager to get into the building. As the door slid open, he walked inside noticing that Doctor Gaster was already waiting for him with his hands clasped behind his back.

"Right on time, I like that, I hate waiting. Come Sans, we have much to do."

Doctor Gaster turned and walked into an elevator, leaving Sans to scramble to catch up before the doors closed. As the two of them stood in an uncomfortable silence, Sans opened his mouth to say something several times, but quickly closed it remembering Papyrus' words just before he left. It was Doctor Gaster that spoke first.

"I see you wore purple like I had asked... The color, it looks good on you."

Sans' face instantly turned a soft blue hue from blushing, and he looked down to the ground, swallowing hard. "Thank you, s-sir."

"You're welcome. You know Sans, it's not every day I meet someone that could quite possibly become my equal in regards to scientific knowledge and talent. You have a lot of potential in you." Doctor Gaster turned his head slightly to look at Sans, who was visibly shaking and rattling once again from nervousness.

"H-heh... I guess you could s-say I'm pretty.... Sans-sational?...." Sans winced the moment he cracked the pun, bracing himself for the doctor's disgust but when none came he slowly looked up and saw him smiling, his hand stifling a soft short laugh.

"Very clever, Sans. You certainly aren't a numb-skull, as living in Snowdin might suggest. You know, because of the cold, not because of the inhabitants there..."

Sans smiled and laughed at the doctor's attempt at a pun, feeling the tension in the small space break instantly. "Good one, sir."

For the rest of the elevator ride the two skeletons discussed the plans for Sans' first day, which included visiting the Core and getting to know the controls there. Sans' soul fluttered the entire time he accompanied the doctor on his daily routine, cracking jokes and slipping in occupational puns here and there. He was just so overjoyed that he was finally able to live out his dream that he barely noticed just how much Doctor Gaster kept his gaze on him.

 

After the day was over and Sans said his goodbyes to Doctor Gaster and all the other lab assistants, in which the doctor called them his "Followers" in jest, he teleported back home. The house was dark, but Papyrus was sitting on the couch, some random monster show playing on the TV. When Sans walked over to him, he noticed he was asleep, his arm propping up his skull.

"Oh, Paps, I told you not to stay up..." Sans walked over to the side of the couch and scooped up Papyrus in his arms. For as tall as he was, he was actually surprisingly light, and there had already been many occasions where Sans had to take Papyrus up to bed so he was use to his size.

In his state of being partially woken up, Papyrus mumbled something against Sans' shoulder, sounding something like "so proud of you". Sans smiled and walked up the stairs, placing his brother in his race car bed softly.

"You're so cute when you're such a sleepy bones. Good night, Paps, I love you..."

Papyrus mumbled back "I love you too, Sans" and shifted in his bed, snuggling up to a small toy bunny that was next to the pillow.

Sans covered him up, kissed the top of his skull, and walked out of the room closing the door softly. He was exhausted from all the work he had done, but couldn't stop smiling at the thought of what a wonderful day he had. He couldn't wait to tell Papyrus about it in the morning.

 

 


	2. The Suprise

**Summary for the Chapter:**

> Sans and Doctor Gaster's working relationship really begins to blossom, but soon it becomes apparent that the doctor wants more than just an apprentice...

"He liked your jokes?! And the puns too??" Papyrus exclaimed while serving up a large stack of pancakes to his brother, who was sitting at a small two person table, sipping slowly at his coffee.

"Yeah, even laughed at a few of them too, which really seemed to surprise the other assistants in the lab..."

Papyrus took a seat in the chair opposite of Sans and began to cut into his own pancakes, taking them neat without any syrup or butter. "I'm really quite surprised! Who would have guessed that he would have such a lousy taste in humor?"

Sans nearly spit out his coffee at Papyrus' sassy remark and laughed as he looked at his brother, who was smirking at him. "Paps!"

"Nyeh heh heh..." Papyrus sipped his steaming black coffee slowly, looking at his brother's shining face with admiration. "You know, Sans. I'm really happy this is working out for you so well, so far. You just look so happy, genuinely happy, and I haven't seen that look on your face for quite some time."

"I am happy, bro. I'm finally doing something that I've wanted to do. I'm not being forced into some crap job that isn't worth my time. I might actually be able to do great things there... Maybe Doctor Gaster and I could, I dunno, invent something important or whatever. I don't care though, as long as I get to be by his side, and work with him on all his projects no matter how trivial they may be. I do hope that someday though I'll be as impactful on monster-kind as he has been..."

Papyrus set his fork down and took Sans' hand within his. He gave it a soft squeeze and smiled brighter. "Those are big dreams, Sans, but if you have the drive, it could become a reality. Like I said before, I believe in you. I always will."

Sans blushed and patted at Papyrus hand, smiling back at him with equal brightness. "Thank you, Paps. You always know how to make me feel good about myself."

"It's what brothers do!!"

Sans swallowed the rest of his coffee, took a few bites of his syrupy pancakes, and noted that would need to be in for work in about an hour. He realized that during their entire conversation he didn't say one pun or joke, but decided that he would give Papyrus a break for one morning since he was being so sweet and encouraging.

When Papyrus started to work on a puzzle in one of his human puzzle books that he found in the dump, Sans took that as his queue to start getting ready for work. He grabbed the outfit he had worn the previous day, in which he had neatly placed it over the treadmill that Papyrus had put in his room some months back (he said it was because he needed to get in shape and that teleporting all around was not going to give him the exercise he needed), and put it on. He grabbed the purple tie as well, but before he put it on he held it in his hand, feeling the softness of the silk between his phalanges. 'I wonder why the doc likes purple so much...' he thought, but shrugged and put it on, pulling one end of the tie down to tighten the loop back around his collar.

After getting dressed, Sans made his way out of his room and leaned against the banister in hallway, calling down to his brother in the kitchen. "Paps, I'm heading out. I'll see you later!"

"Have a great day at work, Sans!"

"You know I will!" And with that Sans teleported out of the house and arrived at the Lab right outside of Doctor Gaster's door. He knocked several times to the tune of 'shave and a hair cut', and the doctor called him in, standing up from his desk as his apprentice arrived a bit early.

"Ah Sans, early today? How fortunate for me. Your shift doesn't start for at least 30 more minutes, but I am glad to see you are taking initiative. Let us make our way down to the basement and see what is on the agenda." Doctor Gaster walked to Sans' side and placed his hands on his shoulders, looking down at him as he towered over him, his glasses reflecting white in the light of the room.

He was so intimidating, so powerful, and Sans blushed blue as the doctor squeezed at his shoulders. "....Uh, s-sure thing, sir."

"That is, unless there is anything you want to talk about before hand. Is there, Sans?" Doctor Gaster cocked his head to the side, leaned forward, and placed one of his hands on his chin, lifting his face to look into his eyes.

Sans sputtered and laughed nervously at the motion, moving backwards slightly, breaking away the doctor's grasp. "N-nope, nothing! I'm just happy to do whatever you want, so let's get to work!"

Doctor Gaster smirked at his remark and steepled his hands together, standing as upright as he possibly could. "Perfect answer. Let's go to work, Sans."

 

For the rest of the week Sans worked as hard as he could, and made sure he did everything the doctor had asked him to do. He got along pretty well with the other assistants, but sometimes some of them gave him the impression that they might be a little jealous of his ability to work so closely with Doctor Gaster. However, being his casual self, he shrugged it off and chalked it up to just being the new guy in town, or Lab in this case.

Despite all the joy Sans felt at his new job, there was one thing that was puzzling and slightly concerning to him; the doctor's constant use of his hands. During their discussions Doctor Gaster would use his hands to elaborate on certain subjects, and at some points would get so animated during his excitement that Sans was almost sure he would fly away. There was even an occasion where he flung his hands so wide part in exclamation that he knocked an empty beaker off the top of a table, and Sans couldn't help but laugh after he used his Blue Magic to stop the glass from shattering on the floor, resulting in the doctor giving him the most startled looking face ever. However, regardless of the situation, eventually the doctor would realize he was being so animated and would straighten himself up, steepling his hands together, a gesture Sans quickly picked up on as a sort of calming mechanism. Doctor Gaster was also incredibly handsy when it came to giving Sans any sort of praise. Rather than just saying something like "Good job" or "Well done", the doctor would physically touch Sans with a skull pat, shoulder squeeze, back rub, what have you. It was a little awkward for Sans at first, not being use to the physical contact of another monster other than his brother, but he quickly got use to that as well, and even began to slightly crave the doctor's touch after he did something he knew would please him.

Their working relationship was definitely flourishing, and before long Doctor Gaster began talking to Sans about possible secret projects, projects that even the other assistants were not aware about. He had to admit, the projects did sound a little out of his range, just starting up and all, but none the less he was still honored that the doctor would even bring him into that part of his world so early on.

 

A few weeks into his apprenticeship at the Lab, Doctor Gaster called Sans into his office just before he was about to take off for the entire weekend. It was the first significant time off Sans had in a long while, and he was thankful for the little bit of break he was about it have as Papyrus had been bugging him for awhile to spend time with him.

As he sat down in the chair opposite the doctor, he smiled at him and held his hands in his lap, swaying his legs slightly back and forth, eager to go home and rest.

"Sans, I know you are anxious to head home for your break, but I wanted to talk with you a moment before you go..." Doctor Gaster got up from his seat and walked over to the front of Sans' chair, sitting on the edge of his front desk, leaning in casually towards him.

At this point Sans was comfortable with the doctor's closeness, and even came to expect it as it was just how he was. He leaned back in his seat with equal "casualness" and smiled up at the doctor in front of him. "What can I help you with, doc?"

Doctor Gaster looked down to Sans with a smirk, clearing his throat slightly. "I wanted to let you know that you have been an absolute pleasure to work with, and you have certainly brought life back into this dull laboratory with your jokes and wit. You can ask any one of my "Followers", I know I was probably quite lifeless and dull before you came here, but you have certainly sparked something deep within me... To be so close to you, to someone so smart and clever, it has been..." Doctor Gaster leaned forward, placing one of his hands on top of Sans' folded hands, his face only inches away,"...intoxicating."

Sans froze, his eyes going wide at the doctor's sudden closeness. He swallowed hard and began to laugh nervously. "Gosh, sir, maybe we could get you some coffee then? Sounds like you may have had too much to drink, heh..."

Doctor Gaster smirked even wider, his other hand moving to Sans' jawbone, fingers curling around his rounded chin, locking his face into place. "No, on the contrary, I think I need more..." With those words he leaned in further, kissing Sans passionately.

A surprised squeak emitted from Sans' throat as he tried to back off, but the doctor had secured his hands tightly, and his grip on his face made it so he couldn't turn away. A second squeak slipped out followed by a soft moan when the doctor parted his mouth and slid his tongue inside Sans', mingling with his forcefully.

With his chest heaving, and his mind panicking, Sans closed his eyes. With a soft "pop" teleported out of the chair to the corner of the room, leaving Doctor Gaster to stumble forward, catching himself on the armrests of the chair. He looked over to Sans, half lidded and hungry, a long semi-transparent purple tongue hanging from his mouth.

'Purple. His favorite color is purple. It's the color of his Magic...' The realization of the situation made Sans start to sweat, and everything all began clicking into place. Doctor Gaster's praise, the constant closeness and touching, the long hours... He... He wanted... Panic rose within him again as he saw the doctor pull his tongue back in and knock the chair out of his way, darting towards towards him swiftly.

"Sans, no wait, st--" Doctor Gaster reached out to grab his apprentice, but before he could do so Sans closed his eyes once again and "pop" disappeared from the room.

  


Landing violently on the floor next to his bed, Sans hissed and clutched at his chest as he panted violently, the situation being too much to currently handle.

"The doc, he... he... Oh fuck..." Sans leaned his forehead on the side of his bed and attempted to calm his breathing, but one thing was preventing him from doing so, the stiffness that had grown in his pants from the encounter. Sans winced his eyes as he tried to will it away, as he didn't want to believe this was all happening, but his concentration broke as a sharp knock rang out at his door.

"Sans? Are you in there? I heard a crash!"

"Y-yeah Paps!! Uhhh, hard landing (fuck)... It happens some times (no it doesn't)! I'm fine though, I'll be out in a moment!" Sans yelled out in an unusually loud wavering voice.

"O....okay, Sans, if you say so. Well I'll be downstairs getting dinner ready!" Sans could hear Papyrus make his way down the steps, and he sighed out in relief that his brother didn't feel the need to enter his room to check on him. He looked down to see that his erection had thankfully gone away, and he stood up on wobbly legs to try to get his mind back in a good place. He was completely unaware at how much time had passed standing there in deep thought until he heard Papyrus call him down for dinner.

"Dinner... Yeah... Coming Paps!" Taking one last deep breath Sans made his way downstairs, putting on a smile to tell his brother about his day over their meal, naturally omitting one small particular detail...

 

 

**Notes for the Chapter:**

> Oh ho ho! Be sure to bookmark if you like this story, more chapters are on the way! Also follow me on Tumblr at esperaqua.tumblr.com!


	3. Breaking Point

**Summary for the Chapter:**

> A fun day with Papyrus isn't going to make Sans forget about what Gaster did to him...

The following day, as promised, Sans joined his brother on a trip to the Snowdin hillside. His mind, however, was far too clouded to truly enjoy any activities. He was so incredibly tired. No matter how hard he tried to sleep, Doctor Gaster's face kept startling him awake.

Suddenly without warning, Sans felt his world flip upside down when a well aimed snowball clocked him directly in the face, knocking him flat on his back. He fought back the urge to sigh as his brother quickly rushed to his side in concern.

"Sans? You didn't even try to get out of the way! Is something the matter?"

Sans looked up at Papyrus as he scooped a bit of snow out of his eye socket, laying there smiling weakly. "N-no, bro... I uh, ice didn't even see it coming. Snow doubt about it, your aim is getting better."

"That first one was awful Sans, but the second one.... Well, equally terrible." Papyrus reached for Sans to offer some help, but Sans groaned and rolled his head to the side, sinking further into the snow. Sensing his brother’s distress he sat down into the snow beside him.

" Come on Sans, I can tell something is wrong, you haven't been yourself today. Please tell me what's going on?"

Sans signed and closed his eyes. "I just didn't sleep well last night, that's all. This one new... project... at the Lab has really got me on thin ice. I mean, don't get me wrong, I love it... But I'm just really thankful for this break."

"Aww Sans, well, we can go home if you wish? I'll make you some noodle soup and you can take the rest of the day off if you want?"

"Naw, Paps, this is your day, I promised you we would have some fun." Sans sat up as he brushed the snow off his hoodie. "Plus, tibia honest, the cold air is really doing me some good. Hotlands is just so... hot. Heh. And stifling sometimes. And the heat just seems to constantly be around you, which I was fine with at first as I just thought it was just his nature. Uh, I mean its nature... ahem, anyways, I'm fine now, that snowball really brought me to my sinuses...", he tapped on the bone to the side of nasal cavity and winked. Papyrus groaned through a smile.

"I don't think you are, but clearly something has happened that you don't want to talk about, and I can respect that. Just know the Great Papyrus is here for you, brother, if you do want to discuss anything!"

Papyrus got up on his feet, grabbed Sans' hand and yanked him up, brushing the snow off his backside. "Now come on... Tag you're it!! Come get me!!" Papyrus ran off cackling with glee and Sans looked at him in awe, shaking his head and chuckling at his brother’s excitement.

"H-hey Paps, wait up!" Sans stumbled forward and ran to catch up to his brother, only occasionally cheating by using his teleportation magic until he was right by his side once again.

 

The day with Papyrus was exhausting, yet fun, but as Sans laid down in his room for the night he began to feel his senses haunted by insomnia. At some points his room seemed blistering hot, but cracking the window open only gave him temporary relief until his body would get shivering cold, leaving his bones to rattle from exposure on top of his mattress. In addition to his temperature fluxing so drastically, his soul felt oddly drained yet energized, causing his blue magic to sputter uncontrollably around him in waves. Occasionally the magic in his eye would flare up without any prompt, causing random items around his room to levitate before crashing back down to the carpet.

Despite how hard he tried to control his malfunctioning magic, it kept coming back around, especially whenever he began to think about Doctor Gaster. He was such an amazingly powerful monster. The thought of his forceful kiss instantly caused the magic around Sans’ pelvis to stir. Sans refused to accept the reality of the situation. He closed his eyes in protest and tried to will away his growing blue erection that strived to form in his boxer shorts.

After a few hours the constant battle with his own body was to much to ignore and Sans gave up trying to fight it. Gritting his teeth together and growling softly, he allowed his magic to take over and fully form, a thick semi-opaque blue cock forming in its wake.

"G-Gaster, what are you d-doing to me..." He moaned out softly, unbuttoning the opening to his boxers, pulling his throbbing blue member out. Giving it a squeeze in his skeletal hand, he gasped and began to softly stroke. The warmth around him began to burn his bones again as another wave of temperature fluctuation washed over him, but he paid it little attention. His mind focused on the Doc. His strength, his praise, his hands... Sans continued to stroke himself at a quicker pace as he thought of it all.  
Sweat began pouring off Sans' skull as he growled out, his eye blazing blue fire as he felt his body getting close, but it simply wasn't happening fast enough. He decided to flip over onto his hands and knees, thrusting his hips forward in sync with his jerks, panting even harder and heavier.

He began to think of different scenarios with the doctor, some where he was touching the Doc, some where the Doc was touching him, some in which they both simply let their magic do the work for them... each scenario resulted in the thought of the most absolutely fantastic pleasure Sans had ever felt in his life, and he craved more.

As the final image of Doctor Gaster's long purple tongue hung out of his mouth, licking at the air only an inch away from Sans' throbbing cock, everything became too much for his body to keep at bay. Waves of pleasure began to crash over his bones, and his body began to shake violently from the orgasmic rush.

Sans growled out followed by a choked sob as his body seized with his final thrust, thick ribbons of semi-opaque gel like cum erupting from the tip of his thick magical cock all over the bed below him. His orgasm was so incredibly intense, draining him of all the pent up magic he had been fighting off for hours. Sans’ world began to spin. He reached out into the dark to try to steady himself, but as up became down the lights in his eyes flickered out and he collapsed onto the bed.

 

For the rest of the weekend, Sans refused to leave his room. Papyrus, ever the concerned brother, constantly attempted to check up on him, but Sans had locked his door, telling Papyrus that he was "feeling sick" and didn't want to "infect" him. Papyrus knew that they couldn't exactly get "sick" as their bodies didn't have the ability to do so, but he eventually eased up after Sans had begged him to leave him alone through soft sobs on the other side of the door. The last time he went to Sans' door was to place a small bowl of noodle soup and glass of water in front of it, knocking gently.

"S-Sans?... I... I brought you some food. I'm just going to set it right here... I... I'm going to bed now, but I love you. I love you so much, Sans. I hope you are feeling better in the morning..." Papyrus placed his hand on Sans' door, then turned and retired to his room, softly closing his door behind him. He listened for a moment, smiling weakly as he heard Sans' door creak open slightly, then shut, letting Papyrus know that his dinner would hopefully not go to waste.

...But it did. Despite how hungry Sans felt, he just didn't have the stomach for it (.......heh......), and his body was weak, nearly drained of all its energy to do anything but lay in bed. Occasionally he would sob through the waves of intense temperature and magic fluctuations, burying himself into his sheets, but soon enough he was kicking them off when things got too intense.

All he really wanted was for whatever all this all was to just stop. He wanted release, but nothing he did gave him any satisfaction. He was too hot, too cold, too hard all the time... He slowly felt like he was dipping into the edge of madness.

**Notes for the Chapter:**

> Follow me on Tumblr at esperaqua.tumblr.com!


	4. Sweet Release

**Summary for the Chapter:**

> When Sans doesn't show up for work on Monday, Doctor Gaster demands he comes in and explains himself...

Sans awoke in a daze to Papyrus knocking on his door, rather urgently.

"Sans? SANS?! Are you ok in there?? Doctor Gaster is on the phone, he said you didn't come into work on time! He's saying he has been trying to call you, but your phone just goes to voicemail. You had me listed as an emergency contact, and he thought this might be an emergency? SANS???"

Groaning out, Sans looked at his clock. It was two hours after his shift was suppose to start. Fuck. Grabbing the cup of water that Papyrus had given him last night, he took a small sip and yelled out weakly. "Tell him I'm not coming in today. I can't, I'm sick."

Sans heard Papyrus relate his statement to Doctor Gaster, but Papyrus quickly retorted back. "Uhh... Sans? He says that is impossible... He says that you had better be in for work in 20 minutes or you may risk.... termination? Sir, please no!"

Sans could hear Papyrus trying to argue with Doctor Gaster over the phone, but eventually the Doc hung up on him, leaving Papyrus a sniffling mess on the other side of the door.

"B-bro... it's ok." Sans said as he forced himself out of bed, putting on a pair of loose fitting shorts and his favorite blue hoodie. He didn't even bother putting on an undershirt. "I-I'm going to... somehow... I'm going to show him how sick I really am. Then I'll be back. It will be alright..."

Closing his eyes, Sans swore under his breath and cursed the Doc for making him do this. Mustering up all the energy that he possibly could, he focused deeply on Doctor Gaster's office, and "pop" left his bedroom.

Though his magic had been on the fritz he was shocked to see he made it to the doctor's office in one piece. Upon entry he had to steady himself on the nearest chair to keep from collapsing. Doctor Gaster sat behind his desk across from Sans’ trembling form. He steepled his hands together and looked his assistant over with intensity burning in his eyes. Sans' legs started to quake under the scrutiny, causing him to drop into the chair with a soft moan.

"Doc, I'm sick... Can't you see? My brother wouldn't lie, and I wouldn't lie to my brother. I've been like this... nearly all weekend. Please, just let me go back home, if any work needs to be done I'll pull in overtime later on once I've... recovered."

Doctor Gaster sighed and stood up from his seat, walking around to the front of Sans just as he had done a few days before.

Sans flinched as he saw the doctor move in front of him, suddenly the temperature in the room seemed to drop and climb all at once. He began to shiver and sweat as he looked up at Doctor Gaster, who had begun to lean forward slightly.

“Sans, do you not know what this is?”

Sans wiped his brow, and gave the doctor a puzzled look. “Uh... a cold?”

Doctor Gaster chuckled and smirked at Sans. “You know very well that we can't get colds. Come now, you are smarter than this.” Gaster moved his hand forward and stroked at Sans' cheekbone. “No, Sans... You are in heat.”

“... What?”

Doctor Gaster chuckled again and knelt in front of Sans. He moved one hand to Sans' rib cage, and began to trace his fingers along the bones connecting to his sternum. “Yes, you heard me correctly. These symptoms you are experiencing are a side effect from being in heat, and clearly it must be your first heat too. I can barely remember my first time, and I certainly didn't have someone to share it with...”

Sans squirmed in his seat and began to pant. The doctor's closeness and touch was setting his body on fire, and he began to feel that familiar sensation of magic forming in his pants once again. A deep blush set into his cheekbones as the doctor's free hand moved down to his slightly exposed hip bones. He began to trace them as well.

“I take it you have already tried to ease your pain, have you not? It did little good, am I correct? A temporary remedy?”

Sans nodded slowly and moaned out when the doctor gripped at his hip bone, his thumb slipping down past the elastic waistband into his more sensitive areas.

“There are two solutions to this... problem. You either wait it out, in which it can go on for about a week if not treated. Or...” Gaster slowly pulled down the band of Sans' pants, his hard thick blue member bobbing out as the loose fabric easily slid fully past his hips. “...I can help you get over your ailments properly. That is, if you desire.”

Sans gasped as he gripped onto the armrests of the chair, his erection straightening to full attention at Doctor Gaster's words.

“Y-yes, Doc... Please h-help me. I can't stand it any more...”

Gaster grinned wickedly and licked at his mouth with his long glistening purple tongue. “Good, Sans. Now you'll need to trust me on this, and promise not to teleport away. Things will get intense, but it will be for your own good. Do you understand?”

Sans nodded and began to pant even harder, wanting to feel the doctor give him relief.

Suddenly the doctor's hand that was placed on Sans' sternum jerked backwards, pulling Sans' blue soul out of his rib cage with the motion. The doctor brought his soul up to his mouth and gave it a long lick with his tongue.

Sans gasped and groaned out, his eyes wide at the feeling, panic and pleasure being blended into one. “Doc! B-be careful with that!”

Doctor Gaster laughed softly as he moved his head downward, his tongue gently lapping at the tip of Sans' throbbing cock. “Don't question my methods, Sans. I know what I'm doing.” All at once the doctor moved forward, taking all of Sans' cock into his mouth.

Sans yelled out and jerked his hips forward, his fingertips digging into the wood of the chair as he groaned and growled out in desperation.

Doctor Gaster began to suck on Sans' hard pulsing cock, his tongue swirling inside of his mouth around the girth of it. Carefully he also began to rub his thumb gently on the middle of Sans' soul, giving it just a little bit of pressure at first to see how much Sans could take. He looked up at Sans to see his head tilted to the side against the chair, his ribcage heaving with deep intense breaths.

That's when the doctor began to pick up the pace, sucking on Sans' cock with such intensity and desire. The hand that was on the top of his hip slowly moved to his spine. Wrapping his long boney fingers around it, he began to stroke it gently, quickly increasing in strength and intensity.

Sans' eyes shot open and he yelled out, his left eye bursting into wispy blue flames. He was so close to the edge, and his body felt like it was boiling in lava.

“Doc!! Please!! I-I'm going to---!”

Doctor Gaster moaned and continued his work, swifter now, suddenly putting much more pressure on Sans' soul as he still rubbed at it's center in his hand.

Suddenly a choked sob escaped from Sans' throat and he arched his back violently, thrusting his hips forward. Warm spurts of semi-transparent blue cum erupted from the tip of his cock, filling Gaster's mouth quickly. Gaster swallowed eagerly, pressing down harder on Sans' soul, squeezing it in his hand.

Sans yelled out and clutched at his chest. Gaster's squeezing sent him into a panic, but as he came the feeling in his body was so intense, like nothing he had ever felt before. All at once, the pain, the burning in his bones, and the malfunctioning feeling of his magic was gone. He looked down with his eyes still blazing to see Gaster cupping his soul in his hands tenderly. He bent down and kissed it softly, his fingertips gently caressing the sides of it as if it was made of glass. He looked up to Sans with a smile.

“Pressure. Pressure often brings out the most of our potential. It causes us some of us to buckle under stress, or realize our greatest possibilities. By applying pressure to your soul and bringing you to your most vulnerable point, I helped your body focus and overcome your ailment.” Doctor Gaster brought his cupped hands onto Sans' rib cage placed his soul back gently. “However, your soul was already quite fragile, so I'm afraid you will need a little bit of rest to heal up the rest of the way.”

The doctor stood up and took Sans' limp body into his arms, his additional phantom hands helping him support Sans' weight. He could feel Sans breathe slowly and moan in his grip, his eyes closed and his body weak from the experience. He walked over to his couch and set Sans upon it, grabbing a blanket, placing it over him with care.

“I'll call your brother to let him know you are ok. You'll be staying here tonight so I can observe you in case there is any... relapse. But for now, rest Sans. Heal.”

Sans weakly looked up at the doctor, smiling faintly. “Thank you, Doc...” He reached out and grabbed at his hand, but it fell short as he passed out, darkness swallowing him whole.

**Notes for the Chapter:**

> Follow me on Tumblr at esperaqua.tumblr.com! I love fanart, and I will post things even if they have a NSFW tag!
> 
> NEW 8/12/16**** I got my first fanart piece!! Done by the amazing Warningsketchycontent, here is a scene from this chapter (NSFW) http://esperaqua.tumblr.com/post/148713158590/nsfw-this-amazingly-hot-and-sexy-fanart-piece . AHHHHH I love it!!


	5. Anything

**Summary for the Chapter:**

> Sans will do anything Doctor Gaster wants... Anything.

When Sans awoke he glanced around the room in a daze, his mind not quite processing where he was in the dim light. His eyes stopped to rest at a large clock on the wall. As his vision came back into focus he noted the time, 11pm. Twelve hours, he had been out for twelve hours.

“Oh, shit. I hope the Doc really did call Papyrus. If he didn't he is going to have a fit.”

Rubbing at his eye sockets, he swung his legs off the couch and stood up weakly. He still felt a little tired, but thankfully the burning / freezing feeling was completely gone, as was the malfunctioning magic that made his bones feel like they were going to explode. He wanted so desperately to get a drink of water or coffee, something to wet his dry mouth, so he decided to venture out into the lab.

Being late at night, he knew no worker would be there as they had all gone home hours ago. As he walked the halls towards the break room, he paused briefly, taking in his surroundings. It was so quiet, eerily so. The only sound that could be heard was the soft hum of electricity that was supplied by the Core, and the faint drip of water somewhere. It all made him shiver and feel quite lonely, like he was the only one left in the world.

Sans laughed nervously at the thought and cupped his hands to the side of his mouth. “Heh... heh. Echo!” The 'echo' bounced around the walls, filling the corridors with his shouted word. As the sound died down he shouted it once more. “ECHO!!”

“... _Echo_...”

Sans yelped out and nearly fell forward as he heard the deep voice behind him. He spun around ready to defend himself, his left eye blazing with blue magic. Doctor Gaster stood in the center of the hallway, smiling slyly down at him. In one hand he held a cup that read “World's Best ~~Boss~~ **Scientist** ”, and in the other he held a rolled up magazine, some sort of monthly publication.

“Jeez Doc, you scared the daylights out of me.”

Gaster laughed and took a sip out of his mug. “Apparently. Good to see you are well enough to be walking around this late at night. Twelve hours of rest, hm? Your body must have needed the sleep.”

Sans laughed. “Well, I guess you could say I was a quite drained.” Instantly a blue flush grew over his cheekbones as he blushed at his own pun.

“Indeed, you were.” Doctor Gaster hummed pleasurably and turned. “Come, let's get you something to drink. Perhaps something... warm.” He chuckled deeply as he walked into the break room, leaving Sans gaping at his possible sexual innuendo in the hallway.

“Y-yeah, sure.” Sans rubbed at the back of his skull and followed Gaster into the room he entered. As he sat down in silence, he watched the doctor pour him a cup of coffee. Electricity hummed softly in the florescent fixtures above the two of them, and as Gaster sat the cup of coffee down in front of him, neither one of them said a word except for a quiet “thank you”. The silence was almost painful, and all Sans could do was avert his eyes and sip at his coffee as he felt Doctor Gaster's eyes stare right into his soul. After a few uncomfortable minutes, Sans couldn't take it any longer. He set his mug down with a loud bang and threw his hands up in the air.

“What is it doc?! What do you want?! You're starin' at me like... like... You're hungry or something. Can you please tell me what is it exactly you want from me? These last few weeks you have been... You've been relentless. Always touching me. Always so damn close. And now earlier... W-what was that?!”

Doctor Gaster calmly looked at Sans, drinking the last of his coffee before setting his mug down on the table. “That was relief from your ailments.”

Sans growled and stood up, knocking over his chair in his dramatic motion. “Yeah, thanks for THAT Doc, not like don't have any weird feelings about THAT now either.”

Gaster cocked his head to the side slightly and looked to Sans, the expression on his face hard to read. “ Weird feelings? Please explain.”

“Damn it, Gaster- I mean, Doc... I-”

Leaning back slightly in his chair, the doctor smiled. “You can call me Gaster if you wish. I'd say formalities at this point are no longer needed.”

Sans rolled his eyes. “... Gaster. Look, I- I don't understand any of this. I'm so confused right now. I mean, you're my boss, right? W-why would you... Why did you... And I've been thinking about what you did on Friday and...”

Gaster sighed and laughed softly, deeply. “Sans, is it all really that confusing to you? Or do you just not want to admit what you know is true?”

“And that is?”

Gaster licked his mouth and smiled wickedly, his hand tracing down his chest. “That you want me just as much as I want you.”

Sans gasped and backed away from the man that sat at the table, tripping slightly over the chair behind him. “Come on, Doc, I would never... WE could never...”

“Why do you think you even went into heat in the first place? Usually a strong emotion or attachment triggers such a thing in monster-kind. You would not have had your symptoms if you didn't feel something from my actions the other day. Do not deny your feelings, your body doesn't lie.”

Sans swallowed hard as he watched Gaster's hand trace along the front of his pants, a growing erection clearly pressing against the fabric, begging to be freed.

“I--- I do want you. I'm just surprised that you would want me. I'm nothing and you are everything. Great and powerful. Me? I'm just some schmuck that found some books at the dump and studied the shit out of them to get a job here.”

“Oh, but you are so much more than that. You have such amazing potential just waiting to be unlocked, and with my help, I can make you great. We can be great together. Now come here Sans, I need your... _assistance_.”

Swallowing hard again, Sans slowly made his way over to Gaster. Kneeling before him, he looked up at him nervously, but Gaster's smile and nod gave him a slight sense of ease. His hand gently trailed across the doctor's pants to the strained bulge, where he curiously traced over its form with the tip of his phalanx. He began to sweat as he heard Gaster's soft moan. Taking a bit of initiative he unbuttoned and unzipped his pants, pulling his glowing purple member out. Sans gasped as he took in the size of it, it was significantly longer than his, but thankfully about the same thickness.

“On my first day you told me to wear purple. You said it was your favorite color. You are quite the sly one, sir, you had me wear it all this time.”

“And you're about to wear a whole lot more of it, too. Take care of me, Sans.”

Groaning gently, Sans leaned forward rolling his blue tongue over the tip of the hard cock in front of him. Suddenly Gaster placed his hand upon his skull, and in a swift motion he pushed down, forcing him to take it all in his mouth. Sans gagged slightly at first, in shock at how far it was inside his mouth, but as he adjusted himself he began to swirl his ecto tongue around the shaft, sucking at it softly. He could feel Gaster begin to buck his hips up to match the motion, and as such Sans picked up the pace.

Gaster stroked his hand on top of Sans' skull occasionally pushing him down, urging him to take on more. Through gritted teeth he began to growl in delight, a sound that sent shivers down his assistant's spine. “Sans, I'm very close. When I tap on your skull I want you to stop sucking and pull it out of your mouth, do you understand?”

Sans nodded as he continued to work faster. Within a minute of two he heard Gaster's moans catch in his throat, and felt an urgent tapping on his skull. Sans didn't want to part, but he knew he had to obey the doc's orders, so he reluctantly pulled back his tongue and took his cock out with a loud pop.

Gaster gasped sharply as ribbons of thick purple cum burst forth from the tip of his swollen cock, coating Sans' mouth and chin thoroughly. He jerked violently a few times in his chair before his back went slack, his eyes closing to process the intense pleasure he had just felt.

Sans laughed as his tongue swirled around his mouth, lapping at the seed the doctor had spilled. “You weren't kidding, doc.”

Gaster slowly opened his eyes and reached out to his assistant, taking his skull into his hands, bringing it forward. His tongue spilled forth and began to swirl with Sans' tongue, the two of them moaning in delight. Together they both cleaned up the remaining seed that had covered Sans' face, and after the job had been finished Gaster pulled Sans up to rest upon his chest in a tight embrace. “Thank you Sans, thank you.”

“My pleasure, doc. It was a delight to return the favor. Like I always say to myself, I will do anything the doctor wants.”

Gaster chuckled as he stroked the top of Sans' head. “Anything hm? That's good to know. I certainly have plans for the two of us. Great plans...”

Great plans... Sans' soul leapt at the thought of it and he grinned widely, nuzzling his face into Gaster's soft turtleneck. The excitement he felt could barely be contained, and he couldn't wait to see what the doc had in store for the two of them.

**Notes for the Chapter:**

> As always, check me out on Tumblr! esperaqua.tumblr.com (occasionally NSFW 18+)


	6. Hostility

**Summary for the Chapter:**

> Sans begins to face a little hostility at work, as one particular assistant begins to show his jealousy.

**Notes for the Chapter:**

> Short filler chapter, more to come very soon! 
> 
> Follow me on Tumblr! esperaqua.tumblr.com

With their new found relationship quickly blossoming, Sans and Gaster worked together even closer nearly every single day, save for precious days off that he spent with Papyrus. Everything seemed to be working out great, but one thing still continued to nag him... That one thing being some of the other assistants in the laboratory...

Sans' close relationship with the Doc certainly turned a few heads, and a few of those heads were not very pleased that he was able to get so far in such a short amount of time. Some were even jealous of the affection Doctor Gaster openly displayed with Sans from time to time.

One particular day, one of the assistants sat next to Sans in the break room. As Sans stirred his coffee, still sleepy from another late night of working, the assistant leaned back and casually began to talk.

“Hey Sans. Long night huh? No sleep? Doctor Gaster really ride you hard last night?”

Sans rolled his eyes and took a sip of his coffee.”Knock it off, alright? It isn't funny, and I know funny.”

The assistant scratched at his chest, smirking to himself. “What's the matter, Sans? Don't want to talk about all the “boning” you two do when we aren't around. We aren't stupid, we know you are his favorite. We just can't explain why a numbskull backwater country monster would even catch the great Doctor Gaster's eye. Nothing like sleeping your way to the top of the ladder, hm? Most of us are still working our asses off doing actual **WORK** , trying to get up a wrung or two.”

Sans looked over at the other monster, his typical smile faltering into a scowl. “Look, I work my ass off too, it's not like I fuck around all day. I work just as hard as you do, maybe even harder. I do a lot of extra work when you guys aren't here.”

“I bet you do. Someone's gotta polish all the doorknobs around here...” The assistant stuck his finger in his mouth and pulled it out with a wet 'pop', laughing viciously.

In a flash Sans was standing over him, grabbing onto his labcoat tightly. He tipped the other monster back in his chair so deeply that he just barely teetered on the edge of falling. The only thing holding him up was Sans' grip.

“Listen asshole, I said knock that shit off, seriously. Our relationship is what it is. I haven't slept my way up to the top, I got to the top by being good and catching his eye with my work. Maybe if you had put in a little more effort you would have been there. You seem to have such issues with the two of us. Why is that? Did you want the Doc all to yourself? Did you want to be the one holding hands with him instead of me? Just what is your problem with us?”  
The other monster tried to regain his balance, but Sans' grip was too strong. “It's just not right, that's all. You don't deserve his affection!”

“Neither do you.” With that Sans let him go, sending him crashing backwards onto the floor. He hissed and scrambled up to his feet. With a lunge the assistant swung his fist at Sans, but Sans dodged quickly by sidestepping. The assistant swung again, but this time Sans caught his fist in his left hand. With a blue flash of light in his left eye, the assistant was knocked back into the hallway, sliding several feet until he stopped against the wall. He looked up at Sans in a state of shock.

“That's a warning. Next time I won't be so gentle. Now get the hell away from me, and stop bothering me. I've got work to do.”

With a “pop” Sans teleported out of the break room, leaving the assistant trembling in the hallway.

 

For the rest of the week, Sans tried to keep mostly to himself, working only on projects that didn't need others, aside from Gaster, to help him on. When Friday came along, he was ready to get a break from all the stares and snickers.

As he was putting away his soiled linens and changing into his regular clothes, Gaster walked into the locker room, taking a seat on the bench next to him. “Do you have any plans tonight?”

Sans looked over to him as he slid his t-shirt over his rib cage. “Not particularly. But I'm really ready to get out of here...”

“Why?... I have noticed you have been a bit distant. Is something wrong?”  
“Naw, doc, don't worry about it. Just some personal stuff I'm dealing with.”

“Sans.”

Sans sighed at Gaster's firm tone and sat down on the bench. “I'm just getting really tired of the other assistants makin' fun of me behind my back. They've been saying that I've slept my way to the top. It just get's really tiresome, ya know? I've really been working hard here, trying to make some real progress, but all they see is some dumb skeleton monster from the sticks.”

Gaster sighed and placed his hand on top of Sans', giving it a gentle squeeze. “You shouldn't worry about them. If anyone is really giving you any trouble, I can let them know. This is my laboratory after all, just say the word.”

“That's just it, I don't need you to fight my battles for me. I'm an adult, I can handle this all myself.”  
“Sans, you shouldn't ever have to handle anything distressing just by yourself... You have me. We have each other. I'll always be by your side when you need me.” Gaster took Sans' hand and brought it up to his mouth, giving it a gentle kiss.  
Sans blushed a little bit and leaned against him, sighing out. “So, uh... Did you have something you wanted to ask me, doc?”  
“Yes. I wanted to ask if you were free tonight to accompany me to dinner. I feel like we may have been missing out on the more... intimate parts of a relationship...”  
Sans laughed and winked at Gaster, removing his hand from his grip and patting his leg. “I dunno doc, I'd say we have been pretty 'intimate' with each other already.”

Gaster scowled and gave Sans a stern look. “That isn't what I meant. I mean.. Intimate. Like something, ahem, romantic. But if you don't want to we can continue to just have our lustful sessions here.”

“I'm just messing with you. I'd love to, it would give me a chance to get to know you a bit more... The man behind the lab coat.”

Gaster smiled and pushed up his glasses. “Splendid. How about I met you by the Waterfall dock at 8pm?”

Sans looked at Gaster with a confused look on his face. “Waterfall? There aren't any restaurants in Waterfall...”

Gaster chuckled and stood up, walking towards the door. “Waterfall dock at 8pm. I'll see you then.”

Sans just shrugged and gathered his backpack. “Sure thing, doc, whatever you want. See ya then.” And with a “pop” he was gone.

 


	7. Dating Start

**Summary for the Chapter:**

> Dating Start! What's better place to go on a date in the underground than Waterfall?....

Sans growled to himself as he searched through his closet, trying to find something appropriate to wear. Who was he kidding, he had no idea what he even **should** wear. Something formal? Casual? He sighed and plopped on his bed, another agitated growl emitting from his chest.

“Brother? Is something wrong?”

Sans looked up to see Papyrus' head peeking through his open door. “Hey Paps. Well I, uh. I guess I'm going out on a... date... tonight. But I don't know what to wear. This is kind of a new thing to me, heh. I don't suppose you could help me put something together, would ya?”

Sans smirked as Papyrus' eyes lit up and a sequel emitted from his mouth. He practically ran into Sans' room over to his closet. “I would love too! You know I, the great and well dressed Papyrus, have excellent fashion sense!” Papyrus rummaged through the closet, but pulled back with a frown. “Sans, you really have nothing to wear, do you?”

Sans chuckled and waved to Papyrus. “I'm sure there is something in there, look closer, go in if you need to...”

Papyrus started to go in, but then backed off turning to Sans with his hands on his hips. “Oh no, I'm not falling for the 'skeletons in the closet' joke trap again. Never again. You know what, grab some black pants and I'll be right back.”

Sans shrugged with a grin as Papyrus stomped off. He grabbed some clean black slacks from his dresser and put them on as his brother instructed.

Papyrus came back in a few minutes later with a few different shirts and vests. After trying a few of them on, most of which were too tight for Sans' large ribcage, they found a smart looking black sweater vest and a royal blue short sleeve dress shirt.

“So, Sans? ...If you don't mind me asking, who is your date tonight?”

Sans coughed and flushed a bit, forgetting that he hadn't exactly told his brother about what had been happening at the lab during lunch breaks and after hours. “Well, uh... It's um. Doctor Gaster...”

Sans winced as he saw his brother's eye sockets grow even wider. He thought for a moment he would protest, but he jumped back in shock as Papyrus squealed out again and hugged onto him tightly.

“Oh my goodness! I am so happy for you! Did you ask him out, or did he ask you out? He must be really impressed with you either way to want to date!”

Sans laughed nervously and scratched at the back of his skull. “Yeah, you could say that. I don't know why, but I'm really nervous about tonight. I mean I work with him every day, and we... uh, have lunch together, but I haven't ever really done anything with him like this. I guess I just need to think like a proton and stay positive!”

“That's the spirit, Saaaaa-- wait. Was that a science pun?”

“Yeah bro, it was.”

“I'll let that one slide, it had a good message. Anyways, I'll leave you alone, I'm sure you have to get going soon since you always love to cut things so close. I hope you have a great night, and have lots of fun!”

Sans watched Papyrus leave the room, and as soon as he left fully he could instantly feel his body threatening to break into a sweat. “Easy there, easy. Deep breaths, not like this is a big deal or anything. Hoo. Ok. Let's do this.”

Standing up and smoothing his sweater vest down he took a deep breath and teleported out of his room to the Waterfall docks.

 

When Sans arrived he noticed no one was there. He wondered if the doc was ok as usually he was the one waiting for him at the lab each morning. After a few minutes of building nerves and bad thoughts, he began to hear a boat approach in one of the caves along the river. He sighed out in relief when he saw the boatman and Gaster drift up to the river bank.

“I deeply apologize for my tardiness. There was a sort of collapse in one of the more direct tunnels, and we had to take a longer route to get here.” Gaster got out of the boat and grabbed a box that had been down at his feet. He then tipped the boatman who left promptly after and turned to Sans. “Shall we?”

Sans nodded and pointed to the box. “What's that?”

“Ah, it's our dinner! I read in a book that taking a meal out to an outdoor location of your choosing and eating there is called a “picnic”. It's suppose to be intimate, and I know just the spot.” Gaster conjured up a pair of hands and traded the box off to the set. He looked down at Sans and smiled, taking his hand in his.

The two of them silently walked into the cave system of Waterfall with Gaster leading the way. Sans looked over to Gaster occasionally with a flushed face. He was wearing a crisply pressed purple dress shirt with a long lavender tie and pressed black slacks. He looked simply amazing.

After a few minutes and several twists and turns through the cave system, they seemed to reach their destination. It was a small watery cove just off the main water system, complete with a small waterfall trickling out of the wall. The dark stone walls were decorated with glowing bioluminescent flora and several echo flowers grew on the edge of the water.

Sans looked around with his mouth agape and Gaster hummed to himself as he opened the box, setting up a blanket and their meal settings.

“Gaster, it's-- it's amazing in here. How did you--?”

“Find the place? In my spare time I like to explore the underground. I like to just walk, and one day I found this place after getting quite lost. It took me a while to find it again, but once I memorized the path, it became quite simple.” Gaster sat down on the blanket and poured two glasses of wine for Sans and himself. “Come, join me.”

Sans walked over to the blanket and sat down slowly, trying to keep his eyes off Gaster, but he found himself staring more than just once.

Gaster offered him the wine, and Sans took it with a shaky hand. “You look good, by the way. Blue is definitely your color.”

Sans sipped at the wine, then looked over to Gaster, smiling softly. “You look good too. It's nice to see you in something other than a lab coat...”

“Yes, well, I do have other sets of clothing, but thank you.” They both paused, looking at each other, and Gaster continued on. “I know we mostly talk of work related topics, but for tonight let's change it up. I am not your boss right now, I am your... Ahem, well, we are on a date. We are dating. Ask me anything you want.”

Sans rubbed the back of his neck and chuckled nervously. “Yeah, ok. So, uh, how long have you been working at the lab? No, sorry. How about, what do you do for fun? Other than explore caves.”

“Fun?” Gaster swirled his wine in the glass and gave it some thought. “Well, I like to study human history , but that's hardly what anyone would call 'fun'. I do have to go to the Dump though to find books on the subject, and I do enjoy sometimes finding odd human trinkets that capture my interest. I can't say that I have a whole lot of 'fun' in my life, though. Except you, of course. Ah that's it. You Sans, are my fun, you bring me the greatest joy.”

Sans snorted into his wine and choked a bit, his face practically lighting up with a blush. “Aw, jeez, doc... You give me entirely too much credit.”

Gaster shrugged and finished off his glass of wine, pouring himself a second. He began to set out their meal which mostly was composed of small sandwiches, crackers, cheese, and cut up crab-apples. As he did so, he looked to Sans with a smirk. “My turn. What do _you_ do for fun?”

“Sleep.”

“That wasn't entirely fair. I get another one.”

“Nope. My turn. What's in those sandwiches? 'Cause I'm hungry.”

“Well 'hungry', they are composed of bread and two types of jelly. I... Don't exactly have much expertise in the kitchen.” Gaster watched as Sans grabbed one and took a bite.

Sans looked at it questioningly, shrugged, and finished it off. He then grabbed a plateful of the other food items in front of them, nibbling on the crackers and cheese, jutting his pinky out as he did so.

After putting a few items on his plate, Gaster looked at Sans with a returned questionable look. “What are you doing?”

“Bein' fancy. When in doubt, pinky out.”

Gaster blinked then suddenly laughed out loudly, his deep laughter filling up the cavern as bounced off the walls. He shook his head and rolled his eyes at Sans, a wide smile plastered on his face. “You're ridiculous.”

After a few minutes of dining in silence, Gaster finished his plate and took sip of wine to wash everything down. His eyes trailed from the cavern onto Sans, whom he caught staring at him once again. He set his glass down onto the rocky surface outside of the blanket. “My turn for another question.”

Sans finished the food on his plate and grabbed his glass. “Well, technically it should be my turn since you asked what I was doing with my pinky, but I'll let you have a free turn, heh.”

“That's very generous of you...” Gaster smirked at him and chuckled. “What are your aspirations in life? To add, what do you hope to accomplish within your life?”

Sans rubbed at the back of his skull and winked. “This sounds like an interview question, but I'll bite.” He looked up at the ceiling deep in thought, then slowly brought his gaze down to Gaster. “Well first and foremost, I want my life to have meaning. I want to do something great, I want to become well known for something. What that is exactly, I dunno... Maybe I'll invent a cool new thing that will help everyone out around here? Or maybe I'll stumble upon learning how to time travel? Ha, I've already got personal teleportation down to a science. But I guess overall? Welp, I aspire to be great. I want to be great for myself. I want to be great for Papyrus. I even want to be great for you. Between you and my brother, you both give me a lot to look forward to in life. Watching him grow, seeing you create, it's all really inspiring and makes me want to do great things for myself and others.”

Sans finished off his glass of wine and set the glass down on the rocky surface next to Gaster's glass. He began to blush as he caught Gaster staring at him, his hand over his chest. He smirked and winked again. “Hope that was a good enough answer. Looks like it really soul-d you, heh.”

Gaster slowly removed his hand and laughed softly, slowly, looking away in embarrassment.

“Doc, I'm up next. You said you wanted to go on a date with me to be more 'intimate', and you take me to this secluded place... Why, are you trying to seduce me? 'Cause we've already dealt with all that, heh...”

“No, I wanted to get to know you better in a place where it could be just you and me. We only have so much time together in a day, and I wanted a bit more. I enjoy your company, your spirit. Your laughter and your terrible humor. I really just wanted to tell you...” Gaster sighed, paused, and looked at the man before him, then looked up at the ceiling. “Sans... Do you think you could ever love me?”

Sans' eye sockets went wide as he looked at Gaster. “W-what did you just say?”

Gaster looked back over to Sans, his eyes softened into an almost sad expression. “Do you think you could over love me?... Because, I think I'm falling in love with you.”

Sans swallowed and looked down at the blanket. “Well, uh... W-who wouldn't love you, right? Smart guy like you? I'm sure lots of people do...” Sans laughed nervously and looked away from Gaster, a trickle of sweat rolling down his forehead.

Gaster crawled over to Sans' side and placed his hand on his lap. He frowned as he felt Sans flinch away at his touch, so he moved his hand back, hurt. “I'm sorry. Forgive me. That was uncalled for. I... I think we should probably call it a night.” Gaster began to stand, but he felt a tug on the back of his shirt.

“...I already love you...”

Gaster stopped and turned to look at Sans. He saw that his eyes almost seemed to be on the brim of tears as he looked down at the ground, and he clutched the blanket below him. “Sans?”

“I love you, Gaster. I have for awhile, but I was just too shy to let you know. The others at work... They make fun of me, they say I'm only with you to get to the top. But what they don't know is that I would still want to be with you, even if I didn't work there anymore. I-- I think about you all the time. I can't get you out of my head. It's like you are always with me, even when you aren't, and it makes my soul feel warm and wanted.” Sans looked up from the ground at Gaster, tears spilling down his cheekbones. “What you did tonight only made that love grow. All of this? You didn't have to, but you did anyways to try to get to know me more. ME me, not apprentice me. And I really wanted to try to get to know you too, but I'm just so... I get so flustered around you sometimes.”

Gaster kneeled back down before Sans and took his face into the palms of his hands. His thumbs moved to his cheekbones, brushing away the tears. Slowly he moved forward and kissed Sans gently, wrapping his long arms around his back, cradling his head. The two of them stayed like that for what seemed like forever, until Gaster parted slightly. “Sans, I love you, too. You make my soul feel the same warmth. Whenever you are near, I can feel it flutter, almost as if it was trying to sync up to yours. I never want to be apart from your side. I want to be with you always.”

A fresh stream of tears rained down from Sans' sockets, but a smile grew on his face as they rolled past his mouth. He moved forward and kissed Gaster this time, his arms reaching up to return the embrace.

The two sat in each others' arms for the rest of the evening, quietly holding one another as they listened to the water trickle out from the cavern wall. They could feel their souls beating as one as they did so, and they both sighed in content as their hands intertwined with each other, neither one of them willing or wanting to let go of the other.

 

 

**Notes for the Chapter:**

> Follow me on Tumblr for updates at esperaqua.tumblr.com !


	8. Something New

**Summary for the Chapter:**

> After recently discovering their mutual love for each other, Sans and Gaster try something new.

“S-- ns...”

“Nnngg, five more minutes Paps, I'm having the best dream...”

“Sans. Sans, wake up, you fell asleep.”

Sans' eyes fluttered open, the world around him blurry at first, but quickly coming back into focus. He was not in his room like he had thought, but was still in the secret cavern in Waterfall. He was in Gaster's arms.

Gaster smiled down at Sans and wiped a small trail of drool away from the side of his mouth, causing Sans to blush deeply. “My stars, you are so cute.”

“Aww, doc, don't say that... How long was I out?”

“About an hour or so. You looked so peaceful I couldn't bare to disturb you. However, I didn't think spending the night here would be the most ideal situation, so I reluctantly had to.”

Sans propped himself up and chuckled lowly, rubbing his neck. “I'm sorry. I hope that didn't spoil the night.”

Gaster stretched out his long legs to the sides of Sans' body and moved his hands to his shoulders, gently massaging his clavicles beneath his sweater. He smiled as Sans moaned and moved into his touch. “It didn't at all. I quite enjoyed just holding you. I would have joined you, but I don't sleep much.”

“That's gotta be rough on you, doc. You have to sleep sometime.”  
“There's just too much work to do.”  
Sans turned in his spot and looked up to Gaster. “You don't need to work all the time, you know. You don't need to put that burden all on yourself. You have employees who would gladly do the work for you.”

“Be that as it may, if I let them do that, then it wouldn't be _my_ work, now would it?”

“Doc...” Sans flipped around onto his knees facing Gaster, his hands moving to his shoulders. “You gotta learn how to relax.” He moved his body onto Gaster's, pressing their ribcages together. He could feel Gaster's soul flutter with passion in his chest, then began to feel his own soul sync up to it. They both looked up to each other slowly as they felt their soul's beat as one.

Sans laughed gently as he leaned in, kissing Gaster softly. His hand slid from his shoulders up to the back of his neck. His fingers began to dance around Gaster's vertebrae, causing a muffled moan to escape his mouth.

Gaster's hands moved down past Sans' chest, coming to rest at the top of his hips. He could instantly feel Sans' blue magic buzzing down below and knew that it was starting to take shape. His purple magic was doing the same. He parted from Sans gently, looking deeply into his eyes. “I can feel your magic stirring.”

Sans panted softly as a soft blue blush re-formed on his cheekbones. “Doc... I. We don't have to... I don't want to ruin the night...”

“And why would this...” Gaster moved his right hand down to the front of Sans' black slacks, palming the growing illuminated bulge softly, “...ruin the night?”

Sans groaned and gently pressed his hips forward, causing Gaster's touch to be more forceful. His magic was fully formed now, and it began to strain against his slacks, begging to be freed.

“On the contrary, I think it would make a perfect end to a perfect night. Wouldn't you agree... my love?” Gaster smirked at Sans and began to unbutton the front of his slacks very slowly, making sure to trace his finger around each button before undoing it. When all the buttons were unfastened, Gaster reached in his slacks and pulled out his thick throbbing blue cock, giving it a gentle tug.

A surprised gasp escaped from Sans' throat as Gaster touched him. He felt his magic hitch and a small bead of precum began to form on the tip of his hard shaft. He gasped again when he felt Gaster's thumb spread it around, making the top glisten in the cavern light.

“Sans, I wanted to ask you something. We have both given each other a fair amount of equal pleasure, but there is something new I wanted to try with you. Would you be willing to try it?”  
Sans looked down to Gaster's hand stroking his cock with gentle teasing strokes, then back up to his face, nodding slightly. “I-if it's with you, I'll try anything...”

Gaster smiled softly and stroked Sans' cheek with his free hand. “I was hoping you would say that. Now, please remove your slacks. I will remove mine as well.”

Sans nodded quickly and did as he was told. He watched in fascination as Gaster began removing his slacks, his thick purple cock bobbing out as he slid them down to the surface below him. Then he began to unbutton his dress shirt, exposing his form to Sans completely for the first time.

Gaster was definitely a skeleton monster like Sans, but his bones were fused together in a way that made his body look more like a solid form. His rib bones were thick and allowed for very little gaps, but his actual ribcage was very long and slender. His arm bones were also on the thicker side, but they had an almost “muscular” shape to them. His leg bones were long and powerful looking. He was truly a magnificent monster, strong and immaculate, a boss type through and through.

“D-doc...you... you're...”

“Fused? Yes... I know. I've always been like this, not exactly an ideal skeletal speci--”

“No. I was going to say amazing. I've never seen anything quite like you... You're so incredibly ...awe inspiring.”

Gaster's face flushed and he walked up to Sans, placing both of his hands on his cheeks. “Thank you for the compliment, Sans. I must admit, I was a bit apprehensive of showing you my form. It's not... typical.”

“I think it's great. Now doc, what did you want to try?”

“Well, I... I wanted to... take you.”

Sans cocked his head to the side and looked at Gaster questioningly. “Take me? Where?”

Gaster laughed and traced his hands down Sans' sweater. “...Enter you... be inside you. The correct term is 'intercourse', but I think in our instance I would prefer to use the term 'make love'.”

Sans' face broke into a bright blue flush and his eyes sockets opened wide. “Oh! Uhh, are you sure about that doc?..I dunno if I'd... be any good at that sort of thing...”

Gaster traced his hands along Sans' arms and clasped their hands together, squeezing gently. “You'll never know if you are good at something unless you try. Are you up for it?”

“Y-yes....”

“Good. Now, I am going to lay down on the blanket, and I want you to climb on top of me. We can go slow. I don't want to hurt you.” Gaster gave Sans' hands one last squeeze and he parted from his side, laying down on the blanket.

Sans took a deep breath and got down on his hands and knees, crawling on top of Gaster. When his pelvis met with Gaster's pelvis he stopped, looking down at him. Sweat began to trickle down his skull.

“Now, concentrate your magic and fill your pelvis even more with it. Not just at the front. Once you feel it is ready, sit up and straddle me. Take my... cock... and lower yourself down onto it. Let your magic surround me.”

“Alright...” Sans closed his eyes and began to focus his magical energy into his pelvis. As he did it began to glow a bright blue, illuminating his bones gently. Slowly he sat up upon Gaster and positioned himself over his thick purple cock. With a low growl he began to lower himself down, the swirling blue magic inside him fully encapsulating Gaster's shaft, causing the doctor to moan loudly in utter delight.

Gaster's hands moved to Sans' hips after he fully lowered himself down upon his pelvis. “Now, look at me, but concentrate on yourself. Keep your focus. I will start slow.” He began to roll his hips up, sliding his throbbing member in and out of Sans' magic with a gentle page.

Sans moaned deeply and moved his hands on top of Gaster's, moving with him as he thrust. “D...doc?... You can go f-faster. Hnnnn, it feel so good. So full, but so good.”

Gaster nodded and began to huff as he moved a little faster, making sure to take notice of Sans' reactions and facial expressions. Sans' magic was so incredibly tight feeling around him, like nothing he had ever really felt before. Sure he had been with other monsters in his lifetime, but Sans was different. Sans was one of a kind, just like him.

Gaster could feel the magical pressure building up within his body, and groaned deeply as he fought it from swelling too fast. But as he moved with Sans and watched his tongue lob out of his mouth hanging lustfully to the side, he winced and moaned. “Sans... I'm going to cum. Will you cum with me?... I want to feel you erupt all over my bones.”

Sans moaned deeply at the thought and nodded weakly. “Y—yeah. I'm close too. Feeling you inside me is making me feel so full, like I have to release. It's almost too much to h-hold.”

Gaster growled and began to buck into his partner even swifter, each thrust purposeful and almost shocking to Sans' senses. All at once Gaster snarled out and gripped onto Sans' hips tightly. “ Haaa! Hrrrk!... Sans!!!” His eye sockets closed tightly as he bucked up roughly, a generous amount of purple ecto seed erupting from the head of his cock, coating the bottom of Sans' ribs and pelvis thoroughly.

As soon as Sans felt the doctor release inside him, he moaned out and arched his back. In a wave of intense pleasure, several ribbons of thick blue cum shot forth, spurting over Gaster's ribs and even reaching his face.

Gaster closed his right eye and hummed in pleasure as he watched Sans jerk and pant on top of him, the blue magic in his pelvis quickly dissipating, along with his ecto cock. Gaster's faded shortly after, leaving them both covered in a fair amount of magical cum.

“Gaster... that was... intense. But incredible. T-thank you....” Sans leaned down on top of Gaster's chest , attempting to return his breathing back to normal.

“It was indeed. Thank you as well.” Gaster smiled and wrapped his arms around Sans tightly. “I love you.”

Sans smiled widely and snuggled into Gaster's embrace. “I love you too. I uh, think we might wanna get cleaned up a bit before we head back home. It would be a sticky situation if we had to explain what the mess was to anyone that may see us before we can change...”

Gaster laughed softly and nodded, stroking the back of Sans' spine. “That would be wise, but for now, let me just hold you before we have to go. A bath and clean clothes can come later. I want to hold you tightly here and now while I can.”

“That sounds like a great plan...”

The two of them sighed deeply, holding each other tightly, once again listening intently to the soft trickling sounds of Waterfall. They were both finally together, so close... Oh what a wonderful end to a wonderful night.

 

**Notes for the Chapter:**

> And for now this is the end of my Sans x Gaster fanfiction. I hope you enjoyed it!
> 
> Follow me on Tumblr at esperaqua.tumblr.com!


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